NEED HELP, OVERHEATING!!!

[Estii]

So. This weekend while on a long trip and while stuck in traffic, I noticed steam rising from under the hood and my temp gauge on full red. After a long ordeal I finally got home. I assumed it was due to a failed thermostat so swapped it out with a OEM part, fired it up and went for a test around the block. Temp gauge went to full red again So now I'm stumped because I checked for leaks from radiator, hoses, water pump, thermostat housing, and found 0. Also something strange is that the car seems to accept only a very little amount of 50/50 coolant, not even half a gallon before full. other points to mention are I checked radiator fan for functionality and found it operational. I checked relays too and found them operational. And lastly, I swapped the stock radiator for an E36 M3 full aluminum model, and the expansion tank too. I have be running EXTREMELY well on this setup for at least 8 months. Even on the longest of trips, say 100 miles, the temp gauge never exceeded the mid point. Any suggestions are much appreciated. I would really prefer to fix this myself then sending to a mechanic to be robbed.